Katy Perry shared a Smurf-tastic secret about Britney Spears at the Smurfs 2 premiere in Westwood, California, on Sunday. Katy told E! News that she makes an appearance on Britney's contribution to the film's soundtrack, "Ooh La La."

Katy explains, "Me and Bonnie McKee secretly did some background vocals on that song. So maybe listen for our voices." McKee is a frequent collaborator with Katy, and she co-wrote hits such as "California Gurls," "Teenage Dream," "Last Friday Night (T.G.I.F.)," "Part of Me," and "Wide Awake."

Katy and Britney met and posed for photos together on the blue carpet. Katy took her grandmother Ann Hudson to the premiere, and grandma wore a T-shirt that read: "I've got the hots for Papa Smurf."

Britney was accompanied to the premiere her two sons Jayden James, who's 6, and Sean Preston, 7. The kids also make cameos in Britney's music video for "Ooh La La."